Selection the cable size means that we need to select the minimum or the smallest cable size. The minimum cable size will be the smallest cable that satisfies the three requirements. However, with experience it will become apparent that the different nature of installations will determine which of the requirements predominate.
CABLE SELECTION PROCEDURE
The cable selection procedures set
out below detail the guidelines to be followed to determine the minimum size of
cable required to satisfy a particular installation condition.
The following three main factors
influence the selection of a particular cable to satisfy the circuit
requirement:
(1) Current-carrying capacity- dependent upon the method of installation and the
presence of external influences, such as thermal insulation, which restrict the
operating temperature of the cable.
To satisfy the current carrying capacity requirements of a
circuit it is necessary to take in to account a number factors. The following
steps describe how this is done:
(a) Determine the current requirements of the circuit
It is the requirements concerning the relationship between the
required by the load connected to the circuit, the type and current rating of
the over current protective device and the current carrying capacity of the
cable.
(b) determine the method of cable installation to be used.
(c) Determine the environmental conditions in the vicinity of
the cable installation such as:
- the ambient air or soil temperature rating factor
- the depth of laying rating factor
- the soil thermal resistivity rating factor
4/ The resulting value of current represents the minimum
current carrying capacity required of the circuit.
(2) Voltage drop-
dependent upon the impedance of the cable, the magnitude of the load current
and the load power factor.
To satisfy the voltage drop limitations of a circuit it is necessary
to take in to account the current required by the load and route length of the
circuit. The following steps describe how this is done:
(a) determine the current (I) requirements of the circuit.
(b) determine the route length (L) of the circuit.
(c) determine the maximum voltage drop (Vd) permitted on the
circuit run.
(d) determine the voltage drop (Vc) in milivolts per ampere
meter (mV/A.m) using equation
Vc = 1000 Vd/L.I
(3)
Short-circuit temperature limited- dependent upon energy produced during the short-circuit
condition.